The present invention relates generally to lift cylinder assemblies, and more particularly to an upper piston rod guide bushing and mounting arrangement therefor.
In a cylinder-piston assembly such as a hydraulic lift cylinder which includes a hollow cylinder having an elongated cylindrical piston rod supported therein for retractable extension outwardly from the cylinder, it is important that proper sealing and guide means be provided adjacent the outer end of the cylinder from which the piston rod extends to prevent binding and fluid leakage during operation. To permit the base of the cylinder to be desirably closed, as by being first fixedly secured to a base block or the like, it is necessary to assemble the vee packings and associated packing expansion means on the lower end of the piston rod and to thereafter insert the piston and associated packing through the upper end of the cylinder. It is important that there be no sharp edge, as an annular groove, that might cut or damage the vee packings. In such cylinder-piston assemblies, and particularly in high pressure lift cylinders, it is conventional to secure a tubular extension to the upper end of a cylinder for supporting a guide bushing which receives the piston rod therethrough in sliding relation, and to maintain the piston in a concentrically spaced relation from the inner surface of the cylinder. The tubular extension has a larger diameter inner cylindrical surface than the main cylinder bore so that annular retainer ring grooves therein will not damage the packings. It is necessary that the cooperating surfaces of the cylinder, piston rod, and guide bushing be machined to close tolerances and fits to prevent the associated packings from extruding into the clearance spaces. The difficulty in the known constructions lies in controlling the eccentricities of the associated upper cylinder ends, the tubular extensions, the guide bushings, and the piston so as to prevent a stack-up of eccentricity tolerances which would cause binding when the piston rod assembly is extended and thereby lead to premature wear and failure of the cylinder-piston assembly.
In addition, such binding as the piston approaches the end of its travel outwardly of the cylinder results in additional friction during subsequent retraction of the piston rod assembly which can introduce into the lift cylinder assembly a resonant vibration during lowering of the load on the upright of a fork lift truck, for example. It is the type of problem which may or may not occur in a given operation and tends to be somewhat unpredictable, but does occur from time to time dependent upon whether resonance is encountered to effect vibration as a result of additional friction under certain conditions of lift height, lowering speed, fork load, length of chain, elasticity, and the like.